1. Field of the Invention:
This invention relates to photographic lenses of long back focal length, and more particularly to photographic lenses of reduced size with a small number of lens elements, while still maintaining good optical performance at the standard angular field, suited to electronic photographic cameras.
2. Description of the Related Art:
Recently, along with the continuous advance in the technology of video cameras, there have been researched and developed increasing kinds of electronic photographic cameras (still video cameras). The still video camera have their effective picture frame made far smaller than that of the single lens reflex camera for 35 mm photographic film, in most cases, of about 11 mm in diagonal. It is also required that, similarly to the single lens reflex camera using silver halide photographic materials, the still video camera employs a reflex mirror as arranged, for example, just ahead the photosensitive unit. Moreover, there are various glass plates such as low-pass and color filters in front of the photosensitive unit. For these reasons, despite the still video camera has a relatively small picture frame, its back focal length must be made considerably long compared with the single lens reflex camera that has no such glass plates.
For example, in the Gauss type photographic lens of standard focal length with an angular field of about 45 to 46 degrees proposed in Japanese Laid-Open Patent Application No. SHO 62-87922, the back focal length in terms of the diagonal length L of the effective picture frame is about 0.9L. In the still video camera, on the other hand, it must be increased to 2.1L.
To admit of such a long back focal length, the inverted telephoto type is necessarily employed in designing the lens system by the front lens unit of strong negative refractive power and the rear lens unit of positive refractive power.
However, as its negative refractive power strengthens, the front lens unit produces increasing negative distortion, spherical aberration and coma. For good correction of these aberrations, the lens structure becomes complicated. The various aberrations left insufficient to correct in the front lens unit may be corrected by the rear lens unit of positive refractive power. For this case, the structure of the rear lens unit becomes complicated likewise as in the front lens unit. Particularly when a great increase of the relative aperture of the lens system is intended to achieve, this tendency becomes prominent. In the past, therefore, the size of the entire lens system tended to increase. As the document concerning this lens arrangement, mention may be made of Japanese Laid-Open Patent Application No. Sho 61-208020.